This invention relates to automatic cruising speed controllers for automotive vehicles, by which the speed of the vehicle is automatically controlled to a target cruising speed in response to control switches such as the set-up switch and the resume switch. In particular, this invention relates to the automatic cruising speed controllers for automotive vehicles, by which the vehicle speed is controlled quickly to the target speed upon driver's operation of a control switch, such that the transient drop or overshoot of the vehicle speed is effectively suppressed. Further, this invention relates to the automatic cruising speed controllers for automotive vehicles, by which the hunting of the vehicle speed and the delay of the response resulting from the smaller than the normal or the larger than normal running resistance of the vehicle are effectively suppressed.
Automatic cruising speed controllers for automotive vehicles are well known by which the target cruising vehicle speed is set by means of a set-up switch, and, upon operation of a resume switch immediately after the acceleration or the deceleration of the vehicle, the vehicle speed is again automatically controlled to the target cruising speed.
For example, Japanese Laid-Open Patent (Kokai) No. 62-299436 discloses an automatic cruising speed controller by which the vehicle speed is controlled to a constant target speed by driving the throttle actuator coupled to the throttle valve, independently of the accelerator pedal operated by the driver of the vehicle. The control is performed in response to the vehicle speed signal and the control switches such as the target vehicle speed set-up switch.
However, in the case of the above conventional device, the throttle actuator is driven in response to the deviation of the actual vehicle speed from the target speed, or in response to the variation of the acceleration, and no special control is performed during the transient period immediately after the operation of the control switches. Namely, even during the transient period immediately after the operation of the control switches, the adjustment of the throttle valve begins only after the deviation of the actual vehicle speed from the target speed, or the variation of the acceleration, is detected. Thus, as shown at (a) in FIG. 13, during the transient period immediately after the set-up time t0, the vehicle speed V first drops deeply below the target speed V0, and returns thereto after a lapse of a substantial length of time. The stable vehicle speed is established thereafter. Further, as shown at (b) in FIG. 13, a substantial length of time elapses before the throttle position (opening degree) A is stabilized to the throttle position A0 corresponding to the target cruising speed V0. The overshoot of the throttle position (opening degree) A immediately after the set-up time t0 gives rise to an abrupt variation of the acceleration of the vehicle, which gives the driver an uncomfortable feeling. Similar problem arises when the constant speed cruising is temporarily suspended upon emergency braking, etc., and the cruising at the previous constant target speed V0 is resumed thereafter.
The conventional automatic cruising speed controller thus has the following disadvantage. Since the target throttle position (opening degree) of the throttle valve is adjusted in response to the deviation of the actual vehicle speed from the target speed, or in response to the variation of the acceleration of the vehicle, the vehicle speed drops or overshoots during the transient period immediately after the set-up of the target vehicle speed or immediately after the cruising control is resumed. This gives the driver an uncomfortable feeling of deceleration and acceleration.
Further, according to the conventional automatic cruising speed controller, the control gain is set at a constant level corresponding to the flat land driving condition, even when the vehicle is running uphill or downhill. Thus, the conventional automatic cruising speed controller has the following further disadvantage. When the vehicle is running uphill, the control gain is set at a value smaller than the optimum level. As a result, the response is delayed. On the other hand, when the vehicle is running downhill, the control gain is set at a too great value, such that the hunting of the vehicle speed may result. In both cases, the driver's comfort is disturbed.